MONDAY, Feb. 18 (HealthDay News) -- The coffee or other caffeinated beverages a woman drinks during her pregnancy might up the odds for a low birth weight newborn or an extended pregnancy, a new study says.

The researchers included data on nearly 60,000 Norwegian women.

"As the risk for having a low birth weight baby was associated with caffeine consumption, pregnant women might be counseled to reduce their caffeine intake during pregnancy as much as possible," said lead researcher Dr. Verena Sengpiel, an obstetrician/gynecologist at the Sahlgrenska Academy of Sahlgrenska University in Goteborg, Sweden.

She believes the findings should also spur a re-evaluation of current recommendations from the American College of Obstetricians and Gynecologists, which advises that a pregnant woman's caffeine intake not exceed two cups of coffee per day.

However, because the study was observational in nature, it can't establish a cause-and-effect relationship between caffeine and low birth weight, Sengpiel stressed. "We cannot say from our data whether caffeine is the specific substance responsible for the fetus being at greater risk of [becoming a] low birth weight infant, nor did we study if these babies actually had special health problems during the neonatal period," Sengpiel said.

The report was published online Feb. 18 in the journal BMC Medicine.

In the study, Sengpiel's team accounted for all sources of caffeine, including coffee, tea, sodas and food including cocoa (such as is found in desserts and chocolate), for almost 60,000 pregnancies tracked by the Norwegian Institute of Public Health.

They found that while caffeine was not linked to premature birth, caffeine from all sources was tied to a higher risk for reduced birth weight.

For example, if an infant's weight is expected to be 7 pounds 15 ounces, every 100 milligrams of caffeine consumed by its mother a day reduced a newborn's weight by almost an ounce, the researchers reported.

According to the Mayo Clinic, the average cup of American-style brewed coffee contains between 95 and 200 milligrams (mg) of caffeine.

Moreover, every 100 mg of caffeine consumed per day increased the length of pregnancy by five hours. And when the caffeine came from coffee (as opposed to other sources) the length of pregnancy was extended eight extra hours, the study authors found.

Given this finding, it is likely that it is not only the caffeine, but something else in coffee that is acting to extend pregnancy, the researchers added.

Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City, said, "Other studies have indicated that caffeine can affect fetal weight, so this is in accord with findings of other studies."

Why caffeine might cause this effect is unclear, she said.

"We do know that caffeine crosses the placenta and the baby is not able to metabolize it very well, [so] it may affect some of the factors associated with growth," Wu theorized.

She advised that women limit the amount of caffeine they consume during pregnancy. The World Health Organization says 300 mg a day, but in the United States the recommended amount is 200 mg a day, she added.

Wu noted that is the amount of caffeine in two small cups of coffee, not a "Starbucks size coffee." There is less caffeine in a cup of tea, or a piece of chocolate, which has about 35 mg of caffeine, she said.